A pulse detonation engine (PDE) is a type of aircraft engine that utilizes impact energy of a detonation wave with a pulsed form, which is produced in a combustion process of a heat cycle, as motive power.
FIG. 1B is a schematic view illustrating an operating principle of a pulse detonation engine. When a mixture gas, which is combined with a fuel and a gas (oxidant such as air), is ignited in a closed end (at a left side in the figure) of a detonation tube, a deflagration is produced. A deflagration changes into a detonation by using a turbulence generator. That is, a combustion state of a mixture gas transits from a deflagration to a detonation under proper conditions. A detonation is defined as a rapid explosion that generates a supersonic pressure wave (shock wave), so called detonation wave, in the detonation tube. Then, extremely higher impact energy of a detonation wave is released from an open end (at a right side in the figure) to the outside of the detonation tube.
FIG. 1A is a schematic view illustrating some physical quantities (pressure P/P0, volume v/v0 and temperature T/T0) of the inside of the detonation tube at a certain time after transition, of a combustion state of a mixture gas, from a deflagration to a detonation. FIG. 1C is a table numerically illustrating pressure P/P0, volume v/v0, and temperature T/T0, at various points of the detonation tube.
In FIGS. 1A to 1C, a location of each point in the detonation tube is expressed as a dimensionless quantity X/L, which is a ratio of a length X from the closed end of the detonation tube to a total length L of the detonation tube; a pressure at each point in the detonation tube a dimensionless quantity P/Po, which is a ratio of a pressure P at each point to a pressure Po of the initial state in the detonation tube; a volume of a detonation wave at each point in the detonation tube a dimensionless quantity v/vo, which is a ratio of a volume v at each point to a volume vo of the initial state in the detonation tube; and a temperature of each point in the detonation tube a dimensionless quantity T/To, which is a ratio of temperature T at each point to a temperature T0 of the initial state in the detonation tube.
In these assumptions, in the vicinity of 0.8 (location: SW) of the detonation tube, pressure of a detonation wave in the detonation tube sharply rises (this phenomenon is called as Neumann spike). In a range between the vicinity of 0.8 and the vicinity of 1 (location: A (the open end)), pressure in the detonation tube remains in an initial state of an ignition process. Also, in a range between the vicinity of 0 (location: the closed end) and the vicinity of 0.4 (location: B in a state subsequent to expansion in the detonation tube), pressure in the detonation tube remains constant. Moreover, in a range between the vicinity of 0.4 and the vicinity of 0.8, pressure of an expansion wave in the detonation tube monotonously increases. Thus, FIG. 1C shows that a detonation instantaneously generates a detonation wave with a pulsed form having extremely higher pressure and temperature.
In such a manner, a detonation wave, which is propagated in the detonation tube at a supersonic speed, with an extremely higher pressure and temperature than those of normal deflagration, is released to the outside of the detonation tube. A heat cycle of combustion in the form of a detonation is not a Brayton cycle (that is, constant pressure process in which combustion is produced with substantially constant pressure), but a Humphrey cycle (that is, constant volume process in which combustion is produced with substantially constant volume). A thermal efficiency of the pulse detonation engine obtained by using the Humphrey cycle is higher than that obtained by using the Brayton cycle in the normal jet engine. That is,-combustion in the form of a detonation provides a higher thermal efficiency than that of combustion in the form of a deflagration.
The pulse detonation engine available to obtain impact energies of detonation waves intermittently generated with the features described above has hidden potentialities which supersedes all propulsion engines such as a turbofan, a turbo jet, a ran jet and rocket (see Japanese Patent Application No. 2001-097814 (Japanese Patent Provisional Publication No. 2001-355515)).